Triaxial Testing in Portsmouth: Shear Strength for Foundation Design

Portsmouth sits on a mix of chalk, London Clay, and thick estuarine alluvium. Getting foundation shear strength wrong here means excessive settlement or bearing failure. We run triaxial tests under BS 1377-8:1990 and BS EN 1997-2 to extract the effective stress parameters designers actually need. Many Portsmouth sites near the harbour or along the M27 corridor have soft silty clays where undrained strength from a simple hand vane just is not enough. A proper consolidated undrained triaxial with pore pressure measurement gives the friction angle and cohesion intercept that make or break a retaining wall design. For deep basements in the city centre, we often combine the triaxial with a CPT test to correlate the undrained shear strength profile before selecting the final foundation type.

A consolidated undrained triaxial with pore pressure measurement gives you the effective friction angle and cohesion intercept that a simple shear vane cannot provide.

Process overview

A common mistake on Portsmouth sites is assuming drained behaviour in low-permeability clays. We have seen contractors specify bearing capacity from quick undrained tests without checking the effective stress path, and the result is a foundation that creeps under load for years. Our lab runs consolidated undrained triaxial with pore pressure measurement as standard on cohesive samples from the Wittering Formation or the Bracklesham Beds. We measure deviator stress at failure, excess pore pressure at each stage, and plot the Mohr-Coulomb envelope from three specimens at different cell pressures. All triaxial tests are run in our UKAS-accredited facility following BS 1377-8, with saturation checked via Skempton B-value before shear. For granular soils from the raised beaches around Old Portsmouth, we run drained triaxial tests at slow strain rates to capture the true effective friction angle without pore pressure artefacts. The data feeds directly into finite element models for quay wall extensions and coastal defence structures.

Triaxial Testing in Portsmouth: Shear Strength for Foundation Design

Local context

Portsmouth has over 40 km of coastline and large areas below 5 metres elevation. Groundwater is high, and pore pressure dissipation during construction is slow in the London Clay and Lambeth Group clays found across the city. If the triaxial test programme does not include pore pressure measurement during shear, the effective stress parameters are guesswork. We have seen quay wall projects where designers used drained parameters for clay fill behind the wall, ignoring the low permeability of the material. The retaining structure tilted within two years. For any Portsmouth project within 500 metres of the tidal zone, consolidated undrained triaxial testing with pore pressure measurement is not optional. It is the minimum standard for a safe foundation design. The same applies to slope stability assessments along the A3(M) cuttings where relic shear surfaces in weathered chalk require residual strength parameters from multi-stage triaxial tests.

Need a geotechnical assessment?

Reply within 24h.

Email: contact@geotechnical-engineering1.com

Reference standards

BS 1377-8:1990, BS EN ISO 17892-8:2018, Eurocode 7 (BS EN 1997-2:2007), BS 5930:2015+A1:2020

Additional services

Consolidated Undrained Triaxial (CU)

Three-stage CU with pore pressure measurement for effective stress parameters. Standard on London Clay and estuarine alluvium from Portsmouth Harbour boreholes.

Drained Triaxial (CD)

Slow shear rate testing on sands and gravels from the raised beach deposits. Determines the true effective friction angle for quay wall and coastal defence design.

Multi-Stage Triaxial

Single specimen tested at three confining pressures for residual strength on remoulded chalk samples from slope stability investigations along the Portsdown Hill road corridors.

Typical parameters

Parameter	Typical value
Test standard	BS 1377-8:1990, BS EN ISO 17892-8:2018
Test type	CU, CD, UU (consolidated undrained, drained, unconsolidated undrained)
Cell pressure range	Up to 1700 kPa (high-pressure triaxial available)
Specimen diameter	38 mm, 50 mm, 70 mm, 100 mm
Saturation check	Skempton B-value ≥ 0.95 for cohesive soils
Strain rate (drained)	0.002–0.01 mm/min for low-permeability clays
Measured parameters	c', φ', cu, E, ν, K0 (with local strain instrumentation)

Quick answers

How much does a triaxial test cost in Portsmouth?

A standard set of three consolidated undrained triaxial tests with pore pressure measurement typically ranges from £1,500 to £2,110, depending on specimen preparation requirements and whether local strain instrumentation is needed for small-strain stiffness measurement.

What triaxial test type do I need for London Clay in Portsmouth?

For London Clay, consolidated undrained triaxial with pore pressure measurement is the standard. The low permeability of London Clay means drained conditions rarely apply during construction, so the effective stress parameters from CU testing with pore pressure measurement give the most reliable input for foundation and retaining wall design.

How long does a triaxial test programme take?

A standard three-specimen CU triaxial programme takes 7 to 10 working days from sample receipt to reporting. Drained tests on low-permeability clays can extend to 14 days due to the slow shear rate required to prevent pore pressure build-up.

Can you test chalk samples from Portsmouth boreholes?

Yes, we test both intact chalk specimens and remoulded chalk for residual strength. Intact chalk requires careful specimen preparation to avoid fissuring, and we run multi-stage triaxial tests to capture the brittle-to-ductile transition that controls slope stability in the Portsdown Hill cuttings.